Radiometric temperature reference

ABSTRACT

A system or component (device) having an electrical characteristic which varies with changes in its own temperature is employed as part of a feedback loop in an operational amplifier circuit. Power applied to the device raises its temperature which in turn changes the value of its electrical characteristic. The output of the operational amplifier circuit forms the input to a second operational amplifier where the polarity of the signal is inverted and fed back to the input of the first operational amplifier circuit in a positive feedback loop. The circuitry associated with the device automatically alters the power supplied to the device causing it to heat or cool until an equilibrium condition is reached, corresponding to a fixed temperature and fixed energy radiation of the device. Any tendency for variation in the temperature of the device produces an almost instantaneous change in the power supplied to the device which thus acts to maintain a constant temperature and a constant infrared energy level radiation from the body.

United States Patent [72] Inventor Leo G. Monford,,]r.

Texas City, Tex. [21] AppLNo. 880,272 [22] Filed Nov. 26, 1969 [45]Patented Apr.20, V [731 Assign Thalln i fi e sse emeri ePre s? by theAdministrator of the National Aeronautics and Space Administration [54]RADIOMETRIC TEMPERATURE REFERENCE 8 Claims, 2 Drawing Figs.

[52] U.S. 219/505 [51] 1nt.C1. 1105b 1/02 [50] Fieldofsearch 219/501,499, 504, 505; 323/22 [56] References Cited UNITED STATES PATENTS2,918,558 12/1959 Evans 219/499 13 2 511m 2 2 3,215,818 11/1965 Deaton219/499 3,448,245 6/1969 Brouneus eta] 219/505 Primary Examiner-BernardA Gilheany Assistant ExaminerF. E. Bell Attorneys-Marvin J. Marnock,Marvin F. Matthews and G. T.

McCoy ABSTRACT: A system or component (device) having an electricalcharacteristic which varies with changes in its own temperature isemployed as part of a feedback loop in an operational amplifier circuit.Power applied to the device raises its temperature which in turn changesthe value of its electrical characteristic. The output of theoperational amplifier circuit forms the input to a second operationalamplifier where the polarity of the signal is inverted and fed back tothe input of the first operational amplifier circuit in a positivefeedback loop. The circuitry associated with the device automaticallyalters the power supplied to the device causing it to heat or cool untilan equilibrium condition is reached, corresponding to a fixedtemperature and fixed energy radiation of the device. Any tendency forvariation in the temperature of the device produces an almostinstantaneous change in the power supplied to the device which thus actsto maintain a constant temperature and a constant infrared energy levelradiation from the body.

PATENTED APRZO Ian FIG] FIG. 2

Leo G. Monford, Jr

INVENTOR ATTORNEYS RADIOME'IRIC TEMPERATURE REFERENCE ORIGIN OF THEINVENTION The invention described herein was made by an employee of theUnited US. Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to the field of testing and measurement of physicalphenomena. More specifically, the present invention relates to means forgenerating or producing a predetermined energy output to form a standardagainst which a variable or unknown energy may be compared.

In the way of example, the present invention relates to the field ofmeasuring instruments such as infrared spectrometers which are employedto examine and measure different bands and frequencies over a spectrumof infrared radiation. Measuring instruments of the type beingconsidered herein have been used in the NASA Earth Resources Program,which has among its objectives, the measurement of physical phenomenonsuch as sea surface temperature; the temperature of heat energyradiation of different type foliage; temperature variations in healthyand diseased crops; and temperature characteristics of varyinggeological formations. It is anticipated that the present invention willhave significant medical applications in that it is readily applicableto systems wherein precise body temperature measurements are required.

The present invention is also anticipated to be employed in the field ofspace travel particularly in view of its small size, light weight, lowpower consumption, simplicity and reliability of construction.Specifically, the devices of the type employed with the presentinvention may be used in systems which directly monitor pointtemperatures of bodies and, with respect to space activities, may beused for thermal mapping of extraterrestrial bodies such as the moon andthe planets.

2. Brief Description of the Prior Art Many conventional instrumentswhich have been employed for the purpose of measuring variable energylevels employ an internal calibration or reference source having afixed, known energy level which provides the means for determining theabsolute value of the unknown energy level being tested or measured. Thereference source or standard employed in conventional testing systemsgenerally include some means for maintaining the energy level output ofthe standard at a fixed value so that measurements being based upon thestandard will remain accurate. A major problem associated with the priorart devices which employ fixed standards is that the corrective responseof the system which supplies the energy required for maintaining thestandard at the predetermined output level is slow enough thatvariations occur in the energy level output of the standard during thecorrective procedures. Thus, conventional standards having energysupplied for the purpose of maintaining a constant level output willvary between an upper and lower energy output level limit which in manycases covers an undesirably broad range. It is evident that variationsin the value of the standard will produce an error in the measured valueof the unknown energy level equal to the error in the standard.

In conventional systems, variations in the output level of the standardare detected by a sensing means, an appropriate change in the outputenergy level of the standard, convey this information to an energysource feeding the standard and then convert the power supplied to thestandard into the energy emitted from the standard. Expensive andelaborate systems have been proposed and employed in the prior art forthe purpose of reducing this correction lag time to thereby reduce theenergy level output variations of the standard. It will be understood,however, that such systems are expensive, difficult to construct and,because of the large number of components which are required, are oftenundependable in operation.

SUMMARY OF THE INVENTION The present invention employs a component,having an electrical resistive characteristic which varies with itstemperature, both as the energy emitting standard and as the sensingelement to produce and maintain a fixed, preselected energy output withvirtually no fluxuation. The component is self-heated until itsresistance matches a preselected, fixed value and is thereaftermaintained at such value by continuous control of the power beingsupplied. It should be noted that the resistance of any such componentused in this circuit is the temperature determining factor. Thisresistance can be measured only from end point to end point of theresistance element.

In one form of the invention, a thermistor is constructed from a smallbody of a suitable metal oxide semiconductor material coated with alayer of solder having a high silver content. Suitable electrical leadsare secured to the device which is then painted with a layer of highemissivity flat black paint which makes the device nonreflective and asclose as practical to a perfect radiator. The painted thermistor is thenencapsulated within a suitable case and is potted within the case with alow-heat absorbing, electrically insulative material. The thermistorhaving the'described construction generates heat when an electricalpower supply is connected across its leads, with the semiconductormaterial acting in such a way that resistance of the device variesinversely with temperature variation.

Other features and advantages of the present invention will be betterunderstood from the following specification, the related drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of theradiometric temperature reference of the present invention; and

FIG. 2 is perspective view, in vertical section illustrating athermistor suitable for use with the radiometric temperature referenceof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, theradiometric temperature reference of the present invention indicatedgenerally at 10 includes a first operational amplifier 11 whose outputis fed through a series resistance R, to a second operational amplifierI2. The output of operational amplifier 12 is in turn fed back throughfeedback line 13 to a variable resistance R at the input amplifier 11forming a positive, system feedback loop. A thermistor R, is connectedbetween the input and the output of operational amplifier 11 forming asubsidiary feedback loop in the first stage of circuit 10 and a resistorR is connected across the second operational amplifier 12 to form asecond subsidiary feedback loop. Capacitors C, and C are connectedbetween ground and the outputs of operational amplifiers ill and 12,respectively, for a purpose to be described hereinafter.

As indicated by the schematic illustrations of FIG. 1, the input signalsto operational amplifiers I1 and 12 are formed between the negative andpositive input terminals of the amplifiers with the positive terminalsbeing connected to ground. It will be understood that the positive andnegative designations are relative references and appropriate changes inthe system may be made whereby the reference voltage is some fixedvoltage value.

With suitable power supplies of 8+ volts and B volts connected to theoperational amplifiers 11 and 12 as indicated in FIG. 1, the operatingtemperature of the thermistor R, is

determined by the value of variable resistor R,. Amplifier 11 operatesin a constant current configuration while amplifier 12 has unity gainand inverts the polarity of its input signal to form positive feedbackthrough the resistor R,. When the circuit is initially energized, withthe thermistor R, cool, the resistance value of R, is less than theresistance value of the thermistor R, which establishes a gain of geaterthan unity for the first stage of the circuit 10. With the gain of thefirst stage greater than unity, the voltage across the thermistor R,increases until it approaches that of the power supply voltage. Theincreased voltage supplied to the thermistor R, heats it, causing acorresponding decrease in its resistance as temperature increases untilthe resistance value of R, is approximately equal to that of R,whereupon the voltage across thermistor R, reaches the value necessaryfor maintaining resistance equilibrium between R, and R,

The two-stage circuitry illustrated in FIG. 1 cooperates to produce thedesired results with any increase or decrease in the temperature ofthermistor R, The second stage circuitry including resistors R and R,cooperate with the operational amplifier 12 to amplify the voltage in anegative direction to produce a correspondingly higher voltage level atvariable resistor R, and the input to operational amplifier 11, with itbeing understood that operational amplifier 12 produces 180 phase shiftin signal appearing at its input to produce the proper polarity at theinput to operational amplifier 11.

It will be appreciated that the illustrated circuit configurationoperates to produce gain approaching infinity whenever the resistancevalues of R, and R, are different. If the temperature of thermistor R,tends to decease to any degree, its resistance value will undergo acorresponding increase due to the inherent action of the thermistor.Under these conditions, the voltage across the thermistor R, willautomatically increase by action of the circuitry associated with thethermistor causing a corresponding temperature increase. Thus, as thethermistor tries to cool down, resistance is caused to increase and thecircuit automatically and very quickly acts to correct the temperaturedecrease resulting in virtually a constant energy level radiation. Thecorrection of any tendency toward temperature variation is almostinstantaneous and for all intents and purposes is so fast that there issubstantially no variation at all in the amount of energy radiated bythe thermistor. This result is directly attributable to the fact thatthe thermistor acts not only as the sensor but also as the heatgenerating source itself.

As will be understood by those having skill in the an, operationalamplifiers of the type illustrated in FIG. 1 are capable of producingvoltages which increase indefinitely tending to create oscillation.Capacitors C, and C, have been provided for the purpose of controllingthe circuit reaction to temperature change to thereby prevent suchoscillation. It will also be understood that the tendency to oscillatein the circuit is an inherent result of the sensitivity of the circuitsince gain of the operational amplifiers and the sensitivity of theseamplifiers controls the sensitivity of the detection mechanism of thecircuit. As a practical matter, the sensitivity of the sensing functionof the circuit is limited only by the limiting characteristics of thecircuit components employed in the associated circuitry of theoperational amplifiers. An additional limiting factor is of course theability of the thermistor R, to act as close as possible to a knowncharacteristic curve.

It should also be noted that the operational amplifiers of the typewhich may be used with the present invention will oscillate due to highgain input and that a suitable RC circuit or other compensating meansmay be required in a standard configuration to prevent oscillation.Undesirable oscillation may also occur because of the high sensitivityof the system to thermal changes. For this reason, it is desirable tothermally isolate the thermistor R, from its environment, increase itsthermal mass or provide appropriate electrical damping to preventoscillations.

LII

Referring to FIG. 2, there is illustrated an exemplary construction forthe thermistor R, which may be used with the present invention. Thethermistor R, includes a block 20 of a suitable, metal oxidesemiconductor having-a resistance value which varies inversely withvariation in temperature of the material. A layer 21 of high heatconductivity material is deposited along the top of the block 20 toprovide a radiating surface. The layer is preferably solder having ahigh silver content which is capable of being adhered to the material 20to provide the desired high heat conductivity. The layer 21 is coveredwith a surface coat 22 of high emissivity, flat black paint whichrenders the radiating device nonreflective and ensures completeradiation from the body. The block 20 and painted metal layer 21 arepotted in a suitable, low heat absorbing, electrically insulatingmaterial 23 and the entire assembly is mounted in a suitable metalcontainer 24 through which insulated leads 25 and 26 extend forconnecting the thermistor R, into the circuit illustrated in FIG. l. [tis therefore apparent that heat generated in the block 20 is conductedby the metal laver 21 and radiated through the paint coat 22.

While the foregoing invention has been described with specific referenceto a thermistor, it will be understood that other devices or componentshaving an electrical characteristic which varies with variations intemperature may be employed. It will further be understood that thecircuit designed around such alternative devices or components must beadapted to provide the desired corrective power input for correspondingvariations in the given electrical characteristic with any variations intemperature. Positive coefficient devices such as resistancethermometers could be used as a temperature reference device if theposition of R, and R, in the circuit were interchanged. Circuit valuessuch as the unity gain of amplifier 12 may also vary depending on actualcircuit characteristics.

Although primarily described herein as a radiated energy reference, itshould be understood that the circuit maintains a constant temperaturetherefore qualifying for use in many other circumstances. It will alsobe appreciated that with appropriate changes in the associatedelectrical circuitry, the output of the standard may be variedsinusoidally or according to any other given time function to provide avariable reference standard.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof, and various changes in the size,shape and materials as well as in the details of the illustratedconstruction may be made within the scope of the appended claims withoutdeparting from the spirit of the invention.

' lclaim:

1. A reference standard comprising:

a. electrical circuit means;

b. power supply means for supplying electrical power to said circuitmeans;

c. heat energy output means included in said circuit means and connectedwith said power supply means for receiving power therefrom, said outputmeans including a variable resistance means having an electricalresistance characteristic which increases in value as the cat energyoutput value of said output means decreases;

d. control means included in said circuit means for changing the powersupplied to said heat energy output means as a predetermined function ofthe value of said electrical resistance characteristic to producepredetermined heat energy output values from said output means, saidcontrol means including an operational amplifier means having a feedbackloop, said control means including means for decreasing the powersupplied to said heat energy output means as said resistance of saidoutput means decreases.

2. The reference standard as defined in claim 1 wherein said operationalamplifier means further includes first and second operational amplifiermeans with the output of said first amplifier means forming the input ofsaid second amplifier means and with the output of said second amplifiermeans 7 being fed back to the input of said first operational amplifiermeans as positive feedback.

3. The reference standard as defined in claim 2 wherein said heat energyoutput means is connected between the input and the output of said firstoperational amplifier means.

4. The reference standard as defined in claim 3 wherein said heat energyoutput means includes a thennistor.

5. A reference standard comprising:

a. electrical circuit means;

b. power supply means for supplying electrical power to said circuitmeans;

c. heat energy output means included in said circuit means and connectedwith said power supply means for receiving power therefrom, said outputmeans having an electrical characteristic which varies in value as afunction of the heat energy output value of said output means; and

d. control means included in said circuit means for changing the powersupplied to said heat energy output means as a predetermined function ofthe value of said electrical characteristic to produce predeterminedheat energy output values from said output means, said heat energyoutput means including a body of electrically semiconductive materialhaving an electrical resistance value which decreases with increases intemperature of said material according to a predetennined function, heatconductive means connected with said body, and means for supplying anelectrical voltage across spaced positions on said body.

6. The reference standard as defined in claim 5 further including:

a. a black coating disposed over said heat conductive means forproviding uniform heat energy radiation and low reflection from saidbody and heat conductive means; and

b. a coating of low heat conductive material surrounding said body andheat conductive means.

7. The reference standard as defined in claim 6 wherein:-

a. said heat conductive means includes a layer of silver solder adheredto said body; and Y b. separate, spaced electrically conductive leadsare electrically connected to said silver solder and to said material insaid body.

8. A reference standard comprising;

a. electrical circuit means;

b. power supply means for supplying electrical power to said circuitmeans;

c. heat energy output means included in said circuit means and connectedwith said power supply means for receiving power therefrom, said outputmeans including a variable resistance means having an electricalresistance characteristic which decreases in value as the heat energyoutput value of said output means decreases;

d. control means included in said circuit means for changing the powersupplied to said heat energy output means as a predetermined function ofthe value of said electrical resistance characteristic to producepredetermined heat energy output values from said output means, saidcontrol means including first and second operational amplifier meanswith the output of said first amplifier means forming the input of saidsecond amplifier means and with the output of said second amplifiermeans being fed back to the input of said first operational amplifiermeans as positive feedback, said control means decreasing the powersupplied to said heat energy output means as said resistance of saidoutput means decreases. decreases.

1. A reference standard comprising: a. electrical circuit means; b.power supply means for supplying electrical power to said circuit means;c. heat energy output means included in said circuit means and connectedwith said power supply means for receiving power therefrom, said outputmeans including a variable resistance means having an electricalresistance characteristic which increases in value as the eat energyoutput value of said output means decreases; d. control means includedin said circuit means for changing the power supplied to said heatenergy output means as a predetermined function of the value of saidelectrical resistance characteristic to produce predetermined heatenergy output values from said output means, said control meansincluding an operational amplifier means having a feedback loop, saidcontrol means including means for decreasing the power supplied to saidheat energy output means as said resistance of said output meansdecreases.
 2. The reference standard as defined in claim 1 wherein saidoperational amplifier means further includes first and secondoperational amplifier means with the output of said first amplifiermeans forming the input of said second amplifier means and with theoutput of said second amplifier means being fed back to the input ofsaid first operational amplifier means as positive feedback.
 3. Thereference standard as defined in claim 2 wherein said heat energy outputmeans is connected between the input and the output of said firstoperational amplifier means.
 4. The reference standard as defined inclaim 3 wherein said heat energy output means includes a thermistor. 5.A reference standard comprising: a. electrical circuit means; b. powersupply means for supplying electrical power to said circuit means; c.heat energy output means included in said circuit means and connectedwith said power supply means for receiving power therefrom, said outputmeans having an electrical characteristic which varies in value as afunction of the heat energy output value of said output means; and d.control means included in said circuit means for changing the powersupplied to said heat energy output means as a predetermined function ofthe value of said electrical characteristic to produce predeterminedheat energy output values from said output means, said heat energyoutput means including a body of electrically semiconductive materialhaving an electrical resistance value which decreases with increases intemperature of said material according to a predetermined function, heatconductive means connected with said body, and means for supplying anelectrical voltage across spaced positions on said body.
 6. Thereference standard as defined in claim 5 further including: a. a blackcoating disposed over said heat conductive means for providing uniformheat energy radiation and low reflection from said body and heatconductive means; and b. a coating of low heat conductive materialsurrounding said body and heat conductive meanS.
 7. The referencestandard as defined in claim 6 wherein: a. said heat conductive meansincludes a layer of silver solder adhered to said body; and b. separate,spaced electrically conductive leads are electrically connected to saidsilver solder and to said material in said body.
 8. A reference standardcomprising; a. electrical circuit means; b. power supply means forsupplying electrical power to said circuit means; c. heat energy outputmeans included in said circuit means and connected with said powersupply means for receiving power therefrom, said output means includinga variable resistance means having an electrical resistancecharacteristic which decreases in value as the heat energy output valueof said output means decreases; d. control means included in saidcircuit means for changing the power supplied to said heat energy outputmeans as a predetermined function of the value of said electricalresistance characteristic to produce predetermined heat energy outputvalues from said output means, said control means including first andsecond operational amplifier means with the output of said firstamplifier means forming the input of said second amplifier means andwith the output of said second amplifier means being fed back to theinput of said first operational amplifier means as positive feedback,said control means decreasing the power supplied to said heat energyoutput means as said resistance of said output means decreases.decreases.